The present invention concerns a steam generator with a main boiler and at least one fluidized bed furnace connected at the flue gas side with the main boiler. The main boiler displays a combustion chamber which is constructed with burners, side walls and a bottom, is equipped with evaporator heating surfaces and which has feedwater preheaters, superheaters and in a given case reheat superheaters connected downstream thereof. Heating surfaces are arranged in the fluidized bed furnace.
Such a steam generator is known from British Patent GB-PS No. 1,582,534. Through the arrangement of the heating surfaces in the fluidized bed furnace, a higher heat transfer can be attained in the region heated by flue gas. In the known steam generator, however, the flue gases of the fluidized bed furnace are fed to the main boiler behind the combustion chamber so that they flow through the second flue gas train or convection train together with the combustion gases of the combustion chamber. Although the heat of the flue gases of the fluidized bed furnace is hereby utilized for the steam generation process, the flue gases do not, however, let themselves be used so favorably for the entire process as when they are fed to the combustion chamber for afterburning, and in the sense of a flue gas recirculation.
A plant, in which the flue gases of the fluidized bed furnace are fed into the combustion chamber of the main boiler, is described in the Journal "VGB Kraftwerkstechnik 59" (1979) pages 105 to 109. In this plant, a combined gas-steam process is carried out. In that case, the heating surfaces present in the fluidized bed furnace are so connected that exclusively the air fed to the gas turbine is heated up in them. The heating surfaces of the water-steam circuit are housed exclusively in the main boiler. Thereby, the disadvantage results that the advantages connected with the arrangement of water or steam heating surfaces in the fluidized bed furnace cannot be exploited by this plant.